134 UNITY AND DIVERSITY IN BIOCHEMISTRY 



., = K,{A) (B) 



whilst that from right to left is : 



V, = K,{C) (D) 



At equilibrium, when v^ = V2, ki{A) (B) is equal to k2{C) (D) and the 

 equilibrium constant of the reversible reaction is written : 



j^ _ (Q jD) 

 (A) (B) 



We can represent a reversible reaction by using small letters for the 

 number of moles of each reactant. 



aA + bB ^cC + dD 



The relation between the free energy change and the equilibrium 

 constant is given by the equation 



{CY{DY 



AF= -RT\nK+ RT In 



{A)^{Bf 



K is the equilibrium constant, that is, the ratio between the product 

 {Cy{D)^ and the product {Ay {By under conditions of thermodynamic 

 equilibrium. The second term permits AF to be calculated for different 

 activities. If these latter are equal to unity, the second term disappears 

 and we have 



AF°= -RTlnK 



In common logarithms, and replacing the gas constant R by its value 

 (1-987 cal/degree/mole), 



-AF° = 4-575 T log K 



C. Free Energy and Electromotive Force 



It is known that one can obtain work from certain chemical reactions by 

 constructing a cell. The maximum work AF can be obtained from a direct 

 and precise measurement, that of the E.M.F. of the cell. The voltage E is 

 proportional to the work done for each electron transferred and if it is 

 known, AF can be calculated from the equation 



AF = -nFE 



in which n represents the number of electrons transported in the reaction 

 and F is the Faraday constant. 



In oxidation-reduction reactions, as we shall see, the measurement of 

 E.M.F. provides invaluable information about the free energy of numerous 

 biochemical reactions. 



